The present invention relates to a liquid crystal display which can simultaneously realize clear black-white images and high-density information images.
As a TN (Twisted Nematic) liquid crystal display has some problems such that a visual angle is narrow and that it is disadvantageous for multiplexing drive, there has been proposed a SBE (Supertwisted Birefringence Effect) liquid crystal display.
However, since the SBE liquid crystal display exhibits yellow or blue images, the following attempts have been made so as to convert the yellow or blue images into the black-white images.
(1) Optical Mode Interference OMI Mode
A retardation (.DELTA.nd) conventionally set at 0.8-0.9 is set to about 0.5-0.6, and a twist angle of liquid crystal molecules is set to a proper value within a range of about 180-300 degrees. Further, other parameters such as polarizing plate configuration and d/p (d=liquid crystal cell gap; p=twist pitch) are set to optimum values. (M. Schadt et al., Appl. Phys. LCH., 50(20), 236 ('87))
(2) Guest Host GH Mode
Each parameter is set to an optimum value so that the SBE liquid crystal display may exhibit a high contrast in a blue mode of negative images, and a large amount (several %) of a dichromatic coloring matter of black is added to a liquid crystal composition, so as to obtain black-white images. (Nikkei Microdevice, p79-, October, 1987)
(3) Phase Plate Mode
On the SBE liquid crystal display developing a high contrast, a non-display SBE cell having a twist direction reverse to that of the SBE liquid crystal display is laminated.
However, these attempts as mentioned above still have the following disadvantages.
In the OMI mode liquid crystal display, it is greatly difficult to simultaneously realize the improvement in definition of the black-white images and the increase in density of the images. Accordingly, it is obliged to slightly sacrifice both the definition and the density of the images, resulting in a slightly bluish coloring of the images.
In the GH mode liquid crystal display, it is necessary to add a large amount of dichromatic coloring matter. As a result, the images are dark, and there is a possibility of the coloring matter being deposited at low temperature. Additionally, the images are basically in the negative mode.
In the phase plate mode liquid crystal display, the manufacturing cost is increased, and the visual angle characteristics are unexpectedly inferior. Furthermore, similar to the GH mode liquid crystal display, the images in the phase plate mode liquid crystal display are basically in the negative mode.